Transparency

We are given a public RSA key that is used in the X509 certificate for the HTTPS connection to a Cryptohack domain.

I solve it in the easy way, and uses a subdomain lookup site. A good one to use is crt.sh, and the query is https://crt.sh/?q=cryptohack.org, or https://subdomains.whoisxmlapi.com/. We can go over the list of subdomains to find the correct subdomain for the solution.

We can instead solve this without using this comprehensive list of subdomains. crt.sh does provide search by certificate fingerprint, but we cannot use this service since we are only given the public key and not the full certificate.

Instead, we will use https://search.censys.io/certificates, which does also index the SHA256 fingerprint of the subject key info field in the X509 certificate - the public key.

The SHA256 fingerprint is simply the SHA256 hash of the DER representation of the public key, so a simple command through openssl should give us what we need.

xxxxxxxxxx
openssl pkey -outform der -pubin -in transparency.pem | sha256sum

From the output, we can query on the censys.io site, and we arrive at the certificate we are interested in, including the CN field - the subdomain. The result is available at https://search.censys.io/certificates?q=29ab37df0a4e4d252f0cf12ad854bede59038fdd9cd652cbc5c222edd26d77d2.

Credits to Robin_Jadoul on Cryptohack for the content above.